The invention related to a method and strip-shaped tensional member for strengthening and/or restoring reinforced or prestressed concrete supporting structures.
It is well known to attach prestressed tensional members to the outside of supporting structures to increase the load carrying capacity (strengthening) or to restore to the original load carrying capacity (restoration) supporting structures made of reinforced or prestressed concrete.
For example, brackets made of steel or reinforced concrete are anchored with dowels to the concrete surface. However, due to a relative shifting (displacement) between the axis of the tensional member and the concrete surface there is an unfavorable shift movement created, which has to be absorbed and transferred by the bracket anchoring; in addition, a bond between the tensional member and the concrete surface cannot be produced.
To avoid disadvantages caused by the shift between the tensional member and the concrete surface, it has been disclosed in a method of the type mentioned in the beginning to later attach, with an adhesive, strip-shaped tension members for strengthening and/or restoring reinforced or prestressed concrete support structures. Such a continuous adhesive joint is especially of significance if cracks are present or could occur in the concrete or further widening of the cracks is to be prevented.
If tensional members are bonded to a concrete surface in a stressed condition, then the adhesive joint becomes stressed over a period of time by the introduction of the shearing (transverse) force as a result of the prestress force. Adhesive agents with the lowest possible deformation modulus are used, which offer a rigid adhesive joint to the greatest extent in its hardened condition to avoid creep losses as a result of this shearing stress on the adhesive joint.
High-strength (high tensile strength), thin, and thereby comparably light strips are preferably employed as tensional members for easy handling. The strips made of high-strength material are usually linear elastic up to the point of fracture; an upper flow level is not present in such high-strength materials, which means an area of near constant force over the path of expansion.
If thusly constructed band-shaped prestressed tensional members cover cracks in the concrete surface, which open up during added stress on the support structure, then there is the danger with the described rigid adhesive Joint that the tensional member is stretched passed the crack edges in the crack area to such an extent that there occurs a sudden fracture in this type of strip without prior indication of elastic deformation.
This lack in ductility is a disadvantage and considerably limits the possible utilization factor of the tension members. Therefore adhesive joint agents must be selected for use in the areas in which cracks are to be covered, which have a specific deformation characteristic themselves, which means a high deformation modulus, so that during opening of the cracks an expansion compensation can take place in the tensional member over a length that is clearly greater than the degree of added widening of the cracks. However, such types of adhesive joint agents with high de formation modulus are not suited for permanent anchoring because of the associated creep losses.
In addition, there is the problem that at both ends of the prestressed tensional members the tie force has to be introduced into the minor structure (substructure). In traditional prestressed concrete structures there is provided a so-called rear suspended reinforcement therefor, which prevents development of cracks immediately behind the anchoring of the tensional member. Such a rear suspended reinforcement is also necessary in later attached prestressed tensional members.
In a known method of the type mentioned in the beginning (WO 97/21009, FIG. 5) there is a tensional member provided with fiber components, which is gluedxe2x80x94while prestressed at its middle area of its lengthxe2x80x94to the concrete surface of a supporting structure. The tensional member is clamped at its two end areas into a clamping device, which is joined to the supporting structure via a ductile deformable transition element. The ductile deformable transition element is a bonding agent, which deformation modulus is substantially higher than that of the adhesive joint agent in the middle area of the tensional member. Since the anchoring of the tensional means at its end areas is effective only during excessive load conditions on the tensional means over the entire middle area, which means only in case of local excessive loads, the danger of fracture in the area of cracks is thereby not a voided.
The object of the invention is to overcome the cited disadvantages and to provide a method for strengthening and/or restoring reinforced or prestressed concrete supporting structures, whereby the danger of a fracture in the areas of cracks is avoided through the avoidance of a shift moment during the direct attachment of the strip-shaped tensional member on the concrete surface, and whereby creep losses are excluded.
This object is achieved according to the invention in that the second deformation modulus is substantially lower compared to the first deformation modulus.
In the prestressed area, which extends over the most part of the length of the tensional member, there axe from the start no shearing forces transmitted via the adhesive joint agent between the tensional member and the concrete surface. Only when expansions and particularly cracks develop in the concrete, then there occurs a transfer of shearing forces in local, small areas between the tensional member and the concrete surface. However, since there is an adhesive joint agent used with comparably soft shearing properties in the middle area of the tensional member, there occurs an expansion compensation in the tensional member over the width of the crack so that, in general, a ductile process is employed.
The extensions at the two end areas of the tensional member, which axe not prestressed, serve as anchoring and rear suspended reinforcement. Since an adhesive joint agent with low deformation modulusxe2x80x94which means a generally rigid adhesive joint agentxe2x80x94is used in the area that is not prestressed, the prestress force is transmitted into the concrete without creep losses to a great extent.
The surface-bonding arrangement of the strip-shaped member prevents the development of a shift moment. A separate anchoring and rear suspended reinforcement is not necessary since these functions are achieved in an especially simple and space-saving fashion by the rigid adhesive connection of the non-stressed end areas of the tensional member with the concrete surface.
The invention relates further to a strip-shaped tensional member for strengthening and/or restoring concrete supporting structures. Starting with a strip-shaped tensional member for strengthening and/or restoring concrete supporting structures that is prestressed in a middle area of its length and which is joined to said concrete surface by means of a first bonding agent with a first deformation modulus, and which joined at its two end areas, which are not in a prestressed condition, by means of a second bonding agent with a second deformation modulus, the novel tensional member is characterized in that the second deformation modulus is substantially lower in comparison to the first deformation modulus. The tensional member may consist of steel fibers, synthetic fibers, or preferably carbon fibers.
Finally, the invention also relates also to a device to carry out the method (process) of a strip-shaped tensional member being arranged on a concrete surface with tensioning devices and end anchoring. This device, according to the invention, is characterized in that the tensioning device is provided with a force introduction body that is joined by positive fit to strip-shaped tensional means, which may be moved by sliding in a longitudinal direction of the tensional means relative to a base body attached to the supporting structure, wherein a tensioning drive may be inserted between the force introduction body and the base body, and wherein at least one spacer may be inserted between the buttress surfaces of the base body, which face one another, and the force introduction body.
This tensioning device, which is preferably disposed at one or preferably at both ends of the middle prestressed area of the tensional member, is able to supply the necessary prestress force on the middle area of the tensional member in a structural and space-saving fashion whereby the two ends of the tensional member, which are outside the force introduction points, are not prestressed.